A simple and reliable calibration method for direct analysis of ground-roasted coffee by portable XRF: an accurate analytical tool for total diet studies

Abstract

Coffee, one of the top ten most globally traded commodities, can be grown in different agricultural regions presenting peculiar characteristics. Thus, the determination of inorganic nutrients in ground-roasted coffee has nutritional relevance and allows authentication as well as tracking of the origin of the product. Analytical methods routinely used to determine inorganic nutrients in food are based on time-consuming steps that involve acid digestion followed by elemental quantification. In this study, analytical strategies for the direct determination of inorganic macronutrients (P, K, Ca, and Mg) in commercial samples of ground-roasted coffee were evaluated by portable X-ray fluorescence spectrometry (pXRF) using two commercial spectrometers. Two calibration approaches were tested: (i) using plant-based certified reference materials (CRMs); (ii) matrix-matched calibration, using samples of ground-roasted coffee as well as the corresponding solid residues resulting from infusion, the spent coffee grounds (SCG). The analyte mass fractions in these samples were determined by the reference method (decomposition using HNO₃ and H₂O₂ followed by ICP OES measurements). In the latter method, calibration models were built using reference values and analytical signals obtained by pXRF. Pressed pellets from the samples (CRMs, ground-roasted coffee, and SCG) were prepared. The samples were irradiated for 50 s using both spectrometers under two experimental conditions: X-ray tube voltage of 15 kV and 50 kV. Calibration models with excellent linearity were obtained (r ≥ 0.90) for all analytes. The proposed matrix-matched calibration method was considered the most accurate approach for elemental determination (mainly P and K) in samples of ground-roasted coffee as well as in the spent coffee grounds, an alternative agricultural fertilizer.